Certain selected catechols have been previously employed in hair coloring but only in association with primary intermediates. Typically, they give rise to pale yellow to beige colors. In this type of application, the catechols are not oxidized, rather they react with the oxidized form of primary intermediates to form colored pigments.
Catechols have also been oxidized to produce hair colorants with hydrogen peroxide in the presence of peroxidase. See Japan Kokais 78: 32,132; 59:161,308; and 61:56,119. See also Japanese Patent J83031-225B.
French Patent 1,164,951 relates to the use of persulfates or any of a variety of other oxidants to color hair. There is no recognition in the patent of the importance of pH control in the system. The patent states that the hair coloring reaction may take place over a wide pH range and may, in fact, take place at whatever pH results from mixing the principal ingredients. The pH is permitted to vary appreciably during the course of hair coloring as is shown in several of the examples.
Dopa and dopamine are disclosed as oxidative hair dyeing precursors in the process of Herlihy, U.S. Pat. No. 4,746,322, wherein the aqueous hair dyeing composition comprises said precursor, an organic compound to assist dye dispersion and an iodate or periodate. The dopa or dopamine dye precursor is present in the aqueous hair dye composition in an amount of from about 1 to about 100 mg/ml, preferably from about 5 to about 25 mg/ml. Dopamine is preferred, according to Herlihy. The iodate or periodate is present in the composition at a concentration of 1 to about 50 mg/ml, while the dispersing agent is present in an amount of from about 0.1 to 30% (wt./vol.). Optionally, a color modifier can be incorporated into the aqueous composition of Herlihy, at a level of from about 0.1 to about 10 mg/ml. pH may be maintained between about 3 to 7 by incorporation of an effective amount of a buffer. According to the patent, the above described aqueous compositions disperse the dye on the hair shaft "with little or no penetration into the hair shaft", Column 2, lines 56-58.
Int. Appl. WO 93/05,759, owned by the assignee of this patent application describes a melanin-forming hair dye, characterized by the in situ formation of dihydroxy indole (DHI) from dopa. Ferricyanide or permanganate are used to oxidize dopa to DHI. There is no teaching or suggestion of persulfate oxidation.
Ammonium persulfate which is the presently preferred persulfate for use in this invention is a well known oxidizing agent as are the corresponding alkali and alkaline earth metal persulfates. All are useful in this invention.